The present invention discloses an improved method to reset the plunger raising lever in a spring actuated device such as a staple gun. The present invention is particularly directed to an improvement of the function of a compact reset spring in a typical type of staple gun mechanism. The present invention further discloses a compact plunger and lever design.
In this type of mechanism an actuating lever pivots in a housing to raise a plunger. The plunger typically but not necessarily comprises a simple flat form. The front end of the lever engages an opening in the flat plunger to raise the plunger. At a predetermined point in the rotation of the plunger the lever front end arcs out of the plane of the plunger to free the plunger to be forcibly moved downward by the action of a power spring.
To reengage the opening in the plunger the lever must rotate downward and translate rearward to pass behind the plunger. The lever must finally translate forward into the plunger opening. The reset action is controlled by a reset spring.
French patent 2477-458 shows a typical staple gun mechanism. The reset spring 23 comprises a substantially vertically oriented compression spring pressing the front of the lever. The spring provides a downward and slight forward bias to the front end of the lever.
U.S. Pat. No. 5,765,742 shows a light duty forward action staple gun. The reset spring is a compression spring above the front end of the actuating lever similar to that of French 2477-458.
UK patent 2032327 shows a substantially identical mechanism to French '458, except that reset spring 12 is an extension type mounted such that it pulls the lever strongly forward and, almost indirectly, the front end downward.
European patent 0281541 shows a somewhat different mechanism from the previous two. Reset spring 15 is identical to that of French '458, specifically a downward and slightly forward pressing compression spring.
U.S. Pat. No. 3,149,339 shows a more complex mechanism where the lever engages the plunger indirectly. Reset spring 49 is a torsion spring wrapped around post 51. The reset spring also functions to bias the plunger engagement.
The reset mechanisms described above are typical of the prior art. They are either too complex or of limited effectiveness. Of particular relevance are the three European patents using the simplified lever. Although such mechanisms are easy to manufacture they do not provide reliable function. In the case of the downward acting compression spring the lever front end readily moves rearward of the plunger and downward. But since the forward bias is gentle, the re-engagement action is weak. In use the lever may prematurely disengage the plunger since the reset spring may not have adequately translated the lever forward into the opening in the plunger. Empirical observation has shown this re-engagement to be weak.
In the case of the forward pulling extension spring the engagement bias is more than adequate. However the downward bias on the lever front end is weak. The lever is prone to hanging up on the top edge of the plunger.
In an optimum design the lever front end is not pressed forward at all through most of the reset motion. In fact a rearward bias to the lever would be desirable. After the lever front end is lowered past the top edge of the plunger a forward bias should occur. The lever then slides smoothly along the rear surface of the plunger and firmly moves forward to engage the opening in the plunger.
A further liability of the above references is that they cannot be vertically compact and sturdy at the same time. The three European references are typical of the prior art wherein the actuating lever front end is substantially spaced above the spring front end. For example in British '327 opening 7 is well above opening 10 along the plunger. US '742 has two vertically separated openings in the plunger with a rib between them. The rib is prone to breakage and must be especially thin if the power spring and actuating lever are nested closely together.
A design using a compact efficient reset spring combined with a vertically short plunger will facilitate a short compact tool housing when used in a conventional rearward action staple gun. When such elements are incorporated into a forward action staple gun as shown in the Figures a compact mechanism is especially helpful to maintain a reasonable hand grip distance. Vertical compactness is particularly important in a heavy duty format staple gun where the travel of the plunger and size of the components are relatively large.